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1 Answer
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The answers to your two questions both come out of the observation that you can not equate force to acceleration. To get them into a form where you can equate them alter the second one to get the acceleration of the satellite due to the planets gravity.

Should I be using Newton's Second Law?
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Peter TamaroffJun 22 '12 at 2:57

Do you know another way? This kind of observation is one of the more useful tools in solving physics problems---especial those asked in school. You have one acceleration and once force, and you say "Aha! Newton had something to say about that. What happens if I ..." With luck it all works out. What you are looking for here is a relationship in which the mass of the orbiter (which you've noted you do not have) does not appear.
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dmckee♦Jun 22 '12 at 3:01

Right. by the way, this is from an introductory course from an university (a basic common course for the exact sciences and engineering factulty.).
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Peter TamaroffJun 22 '12 at 3:04